The invention relates to a high temperature fuel cell. It is known that, during the electrolysis of water, the water molecules are decomposed by electric current into hydrogen H2 and oxygen O2. In a fuel cell, this process takes place in reverse. Through electrochemical combination of hydrogen H2 and oxygen O2 to form water, electric current is produced with high efficiency and, when pure hydrogen H2 is used as combustible gas, without the emission of pollutants and carbon dioxide CO2. Even with technical combustible gases, for example natural gas or coal gas, and with air (which may additionally be enriched with oxygen O2) instead of pure oxygen O2, a fuel cell produces considerably less pollutants and less carbon dioxide CO2 than other forms of energy production which operate with fossil energy sources. The technical implementation of this principle of the fuel cell has given rise to a wide variety of solutions with different electrolytes and with operating temperatures T between 80xc2x0 C. and 1000xc2x0 C.
Depending on their operating temperature T, fuel cells are classified as low, medium and high temperature fuel cells, and these in turn differ in a variety of technical embodiments.
In a high temperature fuel cell stack made up of a large number of high temperature fuel cells (a fuel cell stack also being abbreviated to xe2x80x9cstackxe2x80x9d in the specialist literature) at least one protective layer, a contact layer, an electrolyte electrode unit, a further contact layer, a further interconnecting conducting plate, etc. are arranged in this order under an upper interconnecting conducting plate which covers the high temperature fuel cell stack.
In such a stack, the electrolyte electrode unit includes two electrodes and a solid electrolyte, formed as a membrane disposed between the two electrodes. An electrolyte electrode unit lying between neighboring interconnecting conducting plates together with the contact layers immediately adjoining both sides of the electrolyte electrode unit constitutes a high temperature fuel cell, to which the sides of each of the two interconnecting conducting plates adjoining the contact layers also belong. This and other types of fuel cells are, for example, disclosed by the xe2x80x9cFuel Cell Handbookxe2x80x9d by A. J. Appleby and F. R. Foulkes, 1989, pages 440 to 454.
The performance of the electrodes or of the electrolyte electrode unit of the high temperature fuel cell is one of the factors determining the efficiency of the entire high temperature fuel cell. The essential parameters involved in this are the rates at which the fuel is converted into electrons, ions and reaction products during the electrochemical reaction, the rate at which the fuel is transported to the site of the electrochemical reaction as well as the conductivity for electrons and ions, which are needed for the electrochemical reaction to proceed. The required electron conductivity of the anode is generally achieved using a so-called xe2x80x9cCermetxe2x80x9d containing a framework of metal grains (for example nickel Ni) and a suitable filler to provide ion conductivity. An electron-conductive ceramic is generally used for the cathode, which is also ion-conductive. The two electrodes and the membrane each contain an appropriate electrolyte to provide the ion conductivity of the structure.
There is a substantial problem in achieving sufficient ion conductivity in the material of each electrode. Furthermore, this ion conductivity must be provided throughout the operating time t of the high temperature fuel cell. In order to achieve this in an electrode designed as a cathode, an electrolyte is admixed with an electrically conductive base material. For example, a lanthanum strontium manganate LaxSr(1xe2x88x92x)MnO3 may be used as the base material.
In the cathodes known from the prior art, the electrolyte of the cathode consists of a zirconium dioxide ZrO2 with which a portion of yttrium oxide Y2O3 is admixed. If the electrolyte contains a zirconium dioxide ZrO2 with the admixture of 8 mol % yttrium oxide Y2O3, then at an operating temperature T of approximately 850xc2x0 C., the cathode has a value of about 13.3 xcexa9cm for the ionic resistance. In an operating time t in excess of 1000 hours, this value for the ionic conductivity of the cathode deteriorates to 22 xcexa9cm. If a 10 mol % portion of yttrium oxide Y2O3 is admixed with the zirconium dioxide ZrO2, then the cathode has a higher value of approximately 17.3 xcexa9cm for the ionic resistance. On the other hand, at an operating temperature t of approximately 850xc2x0 C., this electrode material shows no aging behavior as a function of the operating time t, that is to say essentially no impairment of the value for the electrical resistance and therefore the value for the ionic conductivity of the cathode as well.
It is accordingly an object of the invention to provide a high temperature fuel cell that overcomes the above-mentioned disadvantages and that includes a cathode which has a high ionic conductivity for the cathode and substantially avoids impairment of the conductivity for the cathode with increasing operating time t.
With the foregoing and other objects in view there is provided, according to the invention, a high temperature fuel cell having a cathode which comprises at least a first layer and a second layer disposed on one side of the first layer, in which the first layer contains 30 to 60% by weight of a first electrolyte comprising zirconium oxide ZrO2 and at least a portion of scandium oxide Sc2O3, and the second layer comprises substoichiometric lanthanum strontium manganate having the formula LaSryMnO3 in which the sum of x and y is less than 1. In the formula given for substoichiometric lanthanum strontium manganate, x is in the range from 0.6 to 0.90 and y is in the range from 0.02 to 0.39, provided that the sum of x and y is in the range from 0.90 to 0.99. Preferably, x is in the range from 0.65 to 0.85 and y is in the range from 0.08 to 0.30, provided that the sum of x and y is in the range from 0.93 to 0.98.
This second layer promotes the take-off of the electric current I from the high temperature fuel cells. By using scandium oxide Sc2O3 in the first electrolyte of the cathode in accordance with this invention instead of yttrium oxide Y2O3, the value for the electrical resistance of the cathode is substantially reduced (for example halved) in comparison with the cathodes known from the prior art. The ionic conductivity is therefore at least doubled at the same time. Furthermore, the ionic conductivity is substantially constant as a function of the operating time t.
Preferably, the first electrolyte contains 8 to 13 mol % scandium oxide Sc2O3, and it is particularly preferred that the first electrolyte contain 9 to 11 mol % scandium oxide Sc2O3. This range for the scandium oxide Sc2O3 content has been experimentally found to be optimal for improving the ionic conductivity of the cathode.
In a further refinement according to the invention, the first electrode contains approximately 10 mol % scandium oxide Sc2O3. At an operating temperature T of approximately 850xc2x0 C., the ionic resistance has a value of about 6.2 xcexa9cm. Comparison with an electrolyte which contains 10 mol % yttrium oxide Y2O3 instead of scandium oxide Sc2O3 and has an ionic resistance of approximately 17.3 xcexa9cm shows that the ionic resistance is reduced at least by a factor of 2 when using 10 mol % scandium oxide Sc2O3. The first electrolyte containing scandium oxide Sc2O3 shows essentially no increase in ionic resistance as a function of operating time t for at least 1000 hours. The value of the ionic conductivity is therefore improved by at least a factor of 2 in comparison with the cathodes of the prior art.
Preferably, the first layer also contains 40 to 70% of a lanthanum strontium manganate which can be represented by the formula LaxSryMnO3 as the electrically conductive base material for the admixture of the first electrolyte.
In particular, the lanthanum strontium manganate LaSryMnO3 is preferably substoichiometric, that is to say the sum of x and y is less than 1, with the values of x, y, and the sum of x and y within the ranges given above and usually, but not necessarily, identical with the values of x, y, and their sum characterizing the substoichiometric lanthanum strontium manganate of the above-mentioned second layer. Differences if any, between the first layer and the second layer in the values of x and y are preferably small in order to minimize undesired diffusion of lanthanum or strontium between the layers.
By using substoichiometric lanthanum strontium manganate LaSryMnO3 in accordance with this invention, the undesirable formation of lanthanum zirconate is substantially avoided and impairment of the ionic conductivity is therefore prevented.
In a further refinement according to this invention, in the lanthanum strontium manganate LaSryMnO3, x is approximately equal to 0.78 and y approximately equal to 0.20. These values for x and y have proved advantageous in practice. Preferably, the electrolyte contains up to 2.5 mol % aluminum oxide Al2O3. Scandium Sc has virtually the same ionic radius as zirconium Zr, which leads to only minor lattice distortion and consequently to satisfactory ionic conductivity. The stability of this structure is increased even further by the addition of aluminum oxide Al2O3.
When the high temperature fuel cell contains an electrolyte electrode unit which comprises the cathode, an anode and a membrane arranged between the two, the membrane preferably contains zirconium oxide ZrO2 with an 8 to 13 mol % portion of scandium oxide Sc2O3. The membrane of the electrolyte electrode unit, in other words the material at the site of the electrochemical reaction, preferably contains the same components as the first electrolyte of the cathode. The ionic conductivity of the membrane is thereby additionally improved, and the coefficient of thermal expansion is further matched to that of the material of the cathode.
The anode preferably contains 40 to 70% by weight nickel Ni and 30 to 60% by weight of a second electrolyte, which comprises zirconium oxide ZrO2 with an 8 to 13 mol % portion of scandium oxide Sc2O3. The ionic conductivity of the anode is thereby improved in comparison with the anodes known from the prior art.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a high temperature fuel cell, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.